TPMS (tire pressure monitoring system), that is, tire pressure monitoring system, is mainly used to automatically monitor tire pressure and temperature and alarm for tire leakage, too high tire temperature, too low or too high tire pressure. The more advanced TPMS can also inflate automatically to ensure that the motorcycle always runs safely under the standard air pressure. It is an active safety system to ensure the life safety of riders. This paper briefly introduces the important role, composition and key technology of TPMS for the reference of readers.
1 important role of TPMS
The wheel must bear the high-speed alternating compression deformation caused by the mass of the vehicle and the rider; But also bear the impact load and high-frequency vibration caused by uneven road surface; When steering, the circumferential and lateral direction of the tire will also bear distortion and deformation. Repeated deformation will inevitably produce a lot of heat. Modern motorcycle tires made of 4 ~ 6 layers of rubber cord fabric are poor conductors of heat, so it is difficult to dissipate heat inside the carcass. The temperature inside the carcass rises sharply in a short time, the air pressure rises rapidly, and the cord stress increases sharply. If they are violently impacted by obstacles, the tire will inevitably burst, resulting in tire burst in three aspects:
a) Insufficient air pressure will increase the radial deformation of the tire, increase the grounding area, increase the stretching deformation of the cord, and rapidly increase the carcass temperature, which will not only lead to the rapid increase of rolling resistance and fuel consumption, but also easily lead to abnormal wear of the tire shoulder, bending and breaking of the tire sidewall and bursting. If the tire leaks slowly due to wear of the valve or dust during inflation, the tire pressure will be reduced slowly, which is also easy to cause tire burst.
b) The tire pressure is too high, which makes the tire lose elasticity and poor damping performance, greatly reducing the riding comfort; Moreover, only the middle part of the tread is in contact with the road, and the grip decreases, which is very easy to cause abnormal wear and burst of the tire crown. Moreover, it can also expose the latent defects of tires in hot summer or driving for a long time to burst. Such situations are more common.
c) The tire leaks naturally. It is reported that under normal circumstances, the average monthly natural loss of motorcycle tires is about 8KPa. Although it is not a lot, for most riders who do not check their tire pressure habits, they lose as much as 24kpa after three months, equivalent to about 10% of the specified pressure of most motorcycle tires. It can be seen that the slow air leakage of pneumatic tire pressure has a great potential risk of tire burst.
TPMS can monitor tire pressure, temperature and other indicators in real time, and alarm for tire leakage, high tire temperature, low or high air pressure, so that riders can take effective preventive measures in time, eliminate various tire burst accidents in the incubation period, and ensure that the motorcycle is always in a safe state during driving. In addition, TPMS can also prolong the service life of tires and save fuel. The experimental results show that the decrease of wheel pressure by 10% compared with the normal value will lead to abnormal wear of tire shoulder, shorten the service life of tire by 15% and increase the fuel consumption by more than 3%; If the tire pressure is 30% lower than the standard pressure value, resulting in sharp wear of the shoulder, the service life of the tire will be greatly shortened by 50% and the fuel consumption will be greatly increased by 10%. On the contrary, if the tire pressure is too high, it will not only reduce the grip, but also lead to abnormal wear of the tire crown, reduce the tire life and increase the fuel consumption. The increase of fuel consumption not only increases the use cost, but also increases the exhaust emission and environmental pollution.
2 composition and working principle of TPMS
At present, TPMS for motorcycles are mainly divided into two types: active TPMS and passive TPMS. The main difference is that the tire transmitting module in the active TPMS needs battery to provide energy, while the passive TPMS does not need battery.
2.1 composition of active direct TPMS
Active direct TPMS is mainly composed of TPMS transmitting module composed of pressure sensor, temperature sensor, signal processing unit and RF transmitter installed in the tire, and TPMS receiving module 2 composed of RF receiver, digital signal processing unit and liquid crystal display (LCD) installed near the instrument panel.
2.1.1 TPMS transmitting module
It is mainly composed of four parts: intelligent sensor with pressure, temperature, acceleration, voltage detection and post-processing chip combination; Digital microprocessor composed of 4-8-bit single chip microcomputer; High performance and long life lithium battery; Transmit antenna.
The whole transmitting module is fixed in the high-strength ABS plastic shell after packaging, which is not only waterproof, moisture-proof and vibration proof; And small volume (overall dimension: 83mm) × 29mm × 25mm), small mass (40g), wide service temperature range (meeting the automotive service temperature range of – 40 ℃ ~ 125 ℃), working humidity of 100%, emission frequency of 433.92mhz and battery service life of 7 years. The encapsulated launch module can be easily installed on the rim of tubeless tire through the shell. From the appearance, the whole wheel is exactly the same as ordinary tire. The difference is that the wheel equipped with launch module must undergo balance test again (including dynamic balance and static balance) to ensure the ride comfort of motorcycle. In addition, the transmitting module can not be installed in ordinary inner tube tires for the time being.
a) Intelligent sensor
Intelligent sensor is an on-chip system module integrating semiconductor pressure sensor, semiconductor temperature sensor, digital signal processing unit and power manager. Tire pressure is closely related to temperature. According to expert test and analysis: when the tire temperature increases by 1 ℃, the wear increases by 2%. Generally, the temperature cannot exceed 80 ℃. When the temperature reaches 95 ℃, the tire condition is very dangerous. This is the reason that intelligent sensors usually include temperature sensors. In order to strengthen the tire pressure detection function, the intelligent sensor module also adds ASIC digital signal processing units with acceleration sensor, voltage detection, internal clock, watchdog and other functions. These function chips enable the TPMS intelligent sensor not only to detect the changes of tire pressure and temperature during motorcycle driving in real time, but also to realize the functions of immediate start, Automatic wake-up and saving electric energy once the motorcycle is running.
For example, the transmitting module smart sensor in k-press is a system on chip module. Its internal architecture includes a pressure sensor, temperature sensor, accelerometer, battery voltage detection, internal clock integrating silicon micromachining, and an analog-to-digital converter (ADC), sampling / holding (s / h), SPI port, calibration, data management The digital signal processing unit of ID code, the module has mask programmability, that is, it can be configured with customer special software. The intelligent sensor of the whole transmitting module adopts integrated circuit technology, and three independent bare chips of pressure sensor, acceleration sensor and digital microprocessor are made in one package. A pressure / temperature inlet hole is reserved above the package to directly input the tire pressure into the stress film of the pressure sensor. The circular stress film fixed around is composed of semiconductor strain gauge to form a high-precision Wheatstone measurement bridge, and its measurement accuracy can reach 0.01% ~ 0.03% FS. At the same time, the pressure / temperature introduction hole also directly introduces the tire temperature into the semiconductor temperature sensor to detect the changes of tire pressure and temperature parameters at the same time.
b) Processing and transmitting of pressure / temperature signal
The pressure / temperature signal is processed by the circuit in the intelligent sensor module and transmitted to the signal processing unit installed in the transmitting module through its SPI port. It is integrated into a data stream and enters the RF transmitter in the same package. The data is modulated through the amplifier at the specified frequency and transmitted by the antenna. The antenna adopts a ring antenna printed on the printed circuit board. The RF transmission frequency is 315MHz in North America, 433.92mhz in Europe, 448mhz in South Korea and 433.92mhz in Pirelli, Italy. The transmission power is required to output 7 ~ 10dBm at 9.5ma page］
c) Realization of power saving function of lithium sub battery
Since the TPMS transmitting module with lithium sub battery is directly installed in the tire, once installed, the battery cannot be replaced in general. Therefore, it must be ensured that the battery has the same service life as the tire and can work continuously for at least 5 ~ 7 years. Therefore, low power consumption and power saving are very important design indicators.
In order to save power and prolong battery life, designers have taken technical measures to make the system enter sleep mode most of the time. When the motorcycle runs or needs to activate the system, wake up the TPMS system and automatically enter the working state. At present, there are two main technical measures to use the wake-up function: one is to use software to set timing detection. A wake up chip is arranged on the transmitting module, and the receiver sends a start signal; The other is to add an inertial sensor in the intelligent sensor module to automatically enter the system self-test by using the moment of inertia of the tire during operation, and the detection cycle can be automatically adjusted according to the driving speed.
2.1.2 TPMS receiving module
TPMS receiving module structure is mainly composed of RF receiver, signal microprocessor and LCD display. After receiving the IC wireless modulation signal, the RF receiving antenna passes the LAN amplification, LPF filtering and ask / FSK demodulation of the RF analog front end, and the extracted data stream is handed over to the signal microprocessor installed in the same box. After software processing, it restores the tire pressure, temperature and other data of the front and rear wheels for LCD display, intelligently identifies whether the system is safe, and provides audible and visual alarm.
2.1.3 advantages and disadvantages of active direct TPMS
The advantage of active technology is that the technology is mature, and the developed module can be applied to tires of various brands. However, the active TPMS transmitter module needs battery to provide power, so it will inevitably bring some disadvantages. If the battery life is limited; When the temperature drops seriously, the battery capacity will be affected and reduced; This makes the battery reliability unstable. In addition, the chemical substances of the battery will also lead to environmental problems. At the same time, it is difficult to reduce the quality of the launch module due to the existence of the battery. The wheels installed with the launch module must be re weighted and re balance tested.
2.2 composition of passive direct TPMS
The structure of passive direct TPMS is almost the same as that of active direct TPMS, except that there is no battery in the transmitting module, so it is also called battery free TPMS. It uses a central transceiver to replace the RF transmitter in the general active direct TPMS. The transceiver can not only receive signals, but also transmit signals. Its remarkable feature is that the transmitted signal is transmitted by the energy of the received signal. This makes the TPMS transmitting module installed inside the tire do not need a battery to send data, so as to solve the above problems caused by the battery. Although this technology does not use battery power, it needs to integrate the transponder into the tire, which involves the establishment of common standards among tire manufacturers. Therefore, battery free TPMS is difficult to be popular in the short term.
Another passive direct TPMS is the latest technology being developed by Pirelli group of Italy and many European companies. A very advanced surface acoustic wave (SAW) component is adopted in the system. It is reported that saw was discovered by British physicist Rayleigh in 1885, that is, the surface acoustic wave that can emit a certain frequency on the surface of elastic crystal, also known as Rayleigh wave (named after the discoverer).
In recent years, scientists have developed various saw components using saw, which can produce different frequency responses, and are widely used in oscillators, resonators, filters and other circuits in various communication fields. Saw components are highly sensitive. When the crystal is disturbed, the frequency drift is hundreds of kHz. Using the current detection instruments, small changes of 1Hz can be accurately detected. Using these characteristics of saw, researchers place three saw components in each tire, and then use the transmitter to transmit RF signals to saw. When the pressure or temperature in the tire changes, the high-frequency signal transmitted by saw also changes accordingly. After receiving these changed RF signals, the antenna sends them to DSP for processing. The pressure and temperature in the tire can be known according to a specific algorithm. Since saw is a passive device, no battery is required.
2.3 working principle of TPMS
After understanding the composition and structure of TPMS, its working principle is relatively simple. When the motorcycle starts running, the transmitting module installed in the front and rear wheels will wake up automatically. When the system passes the self-test and it is determined that there is no fault, it enters the working state at the same time. The intelligent sensor in the transmitting module automatically detects the tire pressure and temperature data according to the preset program, transmits it to the digital microprocessor for data processing, then sends it to the RF transmitter for wireless modulation, and finally transmits it to the receiving module installed near the motorcycle instrument panel through the transmitting antenna. The receiving module antenna receives the IC wireless modulation signals transmitted from the front and rear wheels at the same time according to the preset program, takes out the data processing, restores the tire pressure and temperature values of the front and rear wheels, directly displays them on the LCD, intelligently identifies whether the system is safe, and provides audible and visual alarm, etc.
3 key technologies of TPMS
TPMS involves many high and new technologies such as sensors, wireless communication, battery life, data reception and display working in harsh environment. The most important key technologies are as follows:
a) Batteries. For active direct TPMS, high-performance long-life battery is one of the key technologies. Once the battery fails, the whole TPMS system will be paralyzed and lose its due function.
Because the battery is placed inside the tire and works in a very harsh environment, it is usually in the temperature range of – 40 ℃ ~ + 125 ℃, while the traditional lithium battery loses power at a low temperature of – 40 ℃ and will automatically discharge at a high temperature of + 100 ℃. Therefore, first of all, the weather resistance of the battery must be solved to ensure that the battery can work normally within the temperature range of – 40 ℃ ~ + 125 ℃. In addition, the battery is generally non replaceable, so it can not use the battery with too large volume and weight, but also must have high reliability and long service life of 7 ~ 10 years.
b) Smart sensors. Intelligent sensors including pressure sensors, temperature sensors and digital microcontrollers are the core of TPMS. Working under the conditions of severe vibration, especially large changes in ambient temperature difference, pressure and humidity and inconvenient for immediate maintenance, they not only require small volume and small mass, but also must have high reliability, high stability, high measurement accuracy and low power consumption. The module design shall select components according to the requirements of military products, formulate strict production process procedures according to the requirements of high-grade electronic products, and 100% acceptance.
c) RF transmitter. At present, there are two working modes of TPMS: ask (amplitude conversion modulation) and FSK (frequency conversion modulation). It is reported that the current main problems focus on RF. Due to the shielding effect of tires on RF and the high-speed rotation of tires, the RF Design of TPMS products will face great technical challenges. This is because the past RF product design is basically in the plane of wireless data transmission. Therefore, it is necessary not only to solve the problem of RF rotary transmission, but also to improve the transmission power and make the transmission power as large as possible, which is one of the keys to ensure the high performance of TPMS. However, the transmission power shall not exceed 10dBm, otherwise it shall be subject to radio control. In addition, the anti-interference performance is good, and the overall size should be as small as possible.
d) Antenna. RF transmitting antenna is the key to improve transmitting power, and RF receiving antenna is the key to improve receiving sensitivity. The nominal sensitivity should reach more than – 100dbm. Antenna technology involves many factors such as antenna geometry, material, medium and so on.
e) Cost. Efforts to reduce costs and improve the performance / price ratio are the key to user acceptance. TPMS for automobile only accounts for a small part of the total cost of automobile. If the price of TPMS for motorcycle is set according to the price of existing TPMS for automobile, it is obvious that motorcycle users cannot accept it, and the TPMS technology for motorcycle will not be developed for a long time.
Responsible editor: GT